Summary Differentiation inhibitory factor (nm23 protein) inhibited the induction of the differentiation of various leukaemic cell lines. We previously reported that nm23 genes (Hi and H2) were overexpressed in acute myelogenous leukaemia (AML) and nm23-H1 expression predicted the prognosis of AML, especially AML-M5. To clarify the correlation between French-American-British (FAB) classification and nm23 expression level and to clarify the involvement of nm23-H2 and nm23-H1 in patient survival, we investigated the relative levels of nm23-Hi and -H2 mRNA in 76 AML samples using the reverse transcriptase-polymerase chain reaction. We confirmed that the expression of both nm23-Hl and -H2 genes in AML samples from three different hospitals was significantly higher than that in normal blood cells (P < 0.0005). Overexpression of nm23-H1 was observed in each FAB AML-M1, -M2, -M3, -M4 or -M5 subtype, and the predictive effect of nm23-Hl expression on AML prognosis was shown in FAB AML-M2 and -M5 cases. Although overexpression of nm23-H2 was also found in each FAB subtype, the expression of nm23-H2 in AML-M1 and -M3 cells was not significantly higher than that in normal cells. Among AML subtypes, AML-M3 showed the lowest expression levels of both nm23 genes. To understand the relationship between nm23-H1 and -H2 expression levels, nm23 expression levels for all the AML cases were plotted and divided into four groups (group A, nm23-H1 and -H2 both high; B, both low; C, only nm23-Hl high; D, only nm23-H2 high). A statistically significant correlation between the levels of expression of nm23-Hl and -H2 was observed (r= 0.726). Most AML-M3 cases belonged to group B, but not other types of AML. Analysis of survival probability between the groups showed that group B survived for significantly longer compared with group A. Furthermore, AML-M3 cases survived for significantly longer compared with non-M3 cases in the same group B. These data suggest that low expression levels of both nm23-Hl and -H2 are associated with good prognosis in AML patients.Keywords: differentiation inhibitory factor; nm23; acute myelogenous leukaemia; acute promyelocytic leukaemia; prognostic factor The degree of differentiation is an important prognostic factor in leukaemia. For example, patients with leukaemia of the undifferentiated phenotype have a lower response rate to treatment and poor survival. Induction of differentiation is closely linked to loss of leukaemogenicity and blocks expression of the malignant phenotypes. Conversely, a disorder of the cellular differentiation of malignant cells reflects clinical behaviour and therapeutic responses. Normal haematopoiesis can be controlled by various positive and negative regulatory molecules. In myeloid leukaemia, these signals continue to operate, but in an unbalanced fashion, allowing emergence and eventual dominance of a malignant clone. Leukaemic cells are arrested in less differentiated stages of development. These results suggest that negative regulators are also important to regulate different...